Ammonia volatilization from flooded systems: A computer model

A computer model is developed to predict NH{dollar}sb3{dollar} volatilization losses from flooded ecosystems. Ammonia volatilization is governed by five primary factors: NH{dollar}sbsp{lcub}4{rcub}{lcub}+{rcub}{dollar}-N concentration, pH, temperature, floodwater depth, and wind speed. Secondary factors modify the primary ones. The model equations are developed as a function of the primary factors consisting of (a) chemical aspects, and (b) volatilization aspects.; An equilibrium exists between NH{dollar}sbsp{lcub}4{rcub}{lcub}+{rcub}{dollar}/NH{dollar}sb3{dollar}(aq) in floodwater which is governed by pH. Ammonium dissociation follows first order kinetics whereas the diffusion controlled association reaction exhibits second order kinetics. A chemical kinetic equation is developed to estimate the volatilization rate of NH{dollar}sb3{dollar}. Expressions are derived to compute the association and dissociation rate constants. Transfer of NH{dollar}sb3{dollar} across the water-air interface is described by the two-film model, characterized by a first order volatilization rate constant. Expressions are developed to compute Henry's law constant, gas, and liquid phase exchange constants. The volatilization rate constant is computed using these constants.; A theory is developed to describe the process of NH{dollar}sb3{dollar} volatilization from flooded systems which states that the rate of NH{dollar}sb3{dollar} loss is principally a function of two parameters, namely, floodwater NH{dollar}sb3{dollar}(aq) concentration and the volatilization rate constant for NH{dollar}sb3{dollar}. Floodwater NH{dollar}sb3{dollar}(aq) is governed by the NH{dollar}sbsp{lcub}4{rcub}{lcub}+{rcub}{dollar}-N concentration, pH, and temperature of floodwater whereas the volatilization rate constant is a function of temperature, floodwater depth, and wind speed.; Model results show that an increase in NH{dollar}sbsp{lcub}4{rcub}{lcub}+{rcub}{dollar}-N concentration, pH, temperature of floodwater, and wind speed increases NH{dollar}sb3{dollar} loss whereas an increase in water depth decreases NH{dollar}sb3{dollar} loss. However, the trend and magnitude of NH{dollar}sb3{dollar} losses differ depending on the conditions existing in the flooded system.; The NH{dollar}sb3{dollar} volatilization model was validated using a wind tunnel to simulate rice paddy conditions and also field experiments. Wind tunnel data show that the model predicts observed values with good accuracy in the range of conditions found in flooded rice systems. The regression of predicted NH{dollar}sb3{dollar} loss on observed losses resulted in an r{dollar}sp2{dollar} of 0.98 and a regression slope of 0.99. Field data also showed very close agreement.